When I draft emails, I always remember the below mnemonic, I read in a magazine.

The ABC of writing – Accuracy, Brevity & Clarity.

After joining work, I learnt a similar mnemonic: KiSS – Keep it Short & Simple)
Then I found this essay Write Simply by Paul Graham. I learnt to write short & simple emails. But my blog posts are very lengthy. Gradually learning to make what I write short & simple.